Coated air-stable cobalt-rare earth alloy particles and method

ABSTRACT

A process for producing novel air-stable coated particles of a magnetic transition metal-rare earth alloy. An organometallic compound which decomposes at a temperature below 500*C is heated to produce a metal vapor which is contacted with particles of a transition metal-rare earth alloy to deposit a metal coating thereon.

United States Patent 191 Smeggil et al.

[ COATED AIR-STABLE COBALT-RARE EARTH ALLOY PARTICLES AND METHOD [75]Inventors: John G. Smeggil, Elnora; Richard J.

Charles, Schenectady, both of NY.

[73] Assignee: General Electric Company,

Schenectady, N.Y.

[22] Filed: Mar. 14, 1974 [21] App]. No.: 451,030

Related US. Application Data [62] Division of Ser. No. 372,691, June 22,1973, Pat. No.

[52] US. Cl... l48/3L57; 117/100 M; 117/107.2 R; 148/103; 148/105 [51]Int. Cl. C0413 35/00; C23C 11/00 [58] Field of Search 148/31.57, 101,103, 105, 148/175;117/10O M,130, 107.2 R; 29/192 CP [56] ReferencesCited UNITED STATES PATENTS 3,385,725 5/1968 Schmeckenbecher 117/130 51July 1, 1975 3,492,175 l/1970 Conrad et al. 148/175 3,591,428 7/1971Buschow et a1 148/3l.57 3,615,914 10/1971 Becker et al 148/101 I OTHERPUBLICATIONS Harwood, J.; Industrial Applications of OrganometallicCompounds, New York, 1963, pp. 88 and 384.

Primary ExaminerWalter R. Satterfield Attorney, Agent, or Firm-Jane M.Binkowski; Joseph T. Cohen; Jerome C. Squillaro [5 7] ABSTRACT 4 Claims,No Drawings 1 COATED AIR-STABLE COBALT-RARE EARTH ALLOY PARTICLES ANDMETHOD This is a division of application Ser. No. 372,691 filed June 22,1973 now U.S. Pat. No. 3,856,580.

The present invention relates generally to the art of 5 making magnets.More particularly, it is concerned with new metal-coated magneticmaterial powders having unique characteristics and a novel method forproducing these coated powders, and with magnets wherein these coatedpowders are the active magnetic components.

Magnetic properties of bulk magnetic materials having largemagnetocrystalline anisotropies can be enhanced by reducing them topowders particularly those having an average particle size of less thanmicrons. The asground powders can be incorporated in bonding media toprovide composite permanent magnets having properties substantiallysuperior to those of the bulk source materials. However, when goodmagnetic properties are attained in the as-ground powders, for example,cobalt-rare earth powders, they tend not to be stable. As the powdersare exposed to air at room temperature and at slightly elevatedtemperatures, their intrinsic coercive force, H which is a measure ofamagnets resistance to demagnetization, decreases irreversibly.Specifically, these magnetic powders are quite reactive to oxygen andwater vapor in the atmosphere at room temperature, and they are evenmore so reactive at even slightly elevated temperatures, i.e., about100C, resulting in a significant loss in their intrinsic coercive force.Thus, a comparatively low value of intrinsic coercive force cansubstantially diminish the advantages to be gained by converting thebulk body to a powder or producing the powder by some other technique,and fabricating a composite finished article from the powder.

The art has used sintering to produce magnets with substantially stableproperties from these powders. This process comprises compacting thepowder to form a green body and sintering the body at high temperatures,generally about 1000C, in an inert atmosphere to produce a high densitycompact having a closed pore structure. Such a structure protects themagnet from the atmosphere resulting in long term stability of itsmagnetic properties. However, this method is expensive, since itrequires power-consuming equipment and handling procedures which aretime-consuming.

A more desirable approach to the fabrication of mag nets using thesepowders, for example cobalt-rare earth alloy powders, would delete thesintering process and merely compact the aligned particles into thedesired shape with the aid of some kind of binder. However, to do thisrequires the use of air stable, accordingly coated, cobalt-rare earthalloy particles.

Attempts to provide cobalt-rare earth alloy powder with a protectivemetal coating deposited from metal vapor of a molten metal haveyieldedlimited success. For example, temperatures of 500C and highersignificantly deteriorate the magnetic properties of the loose powder.Such a method, therefore, can utilize only a very few low meltingmetals, which also must produce sufficient vapor pressures for effectivecoating deposition at temperatures not much higher than their meltingpoint, such as lead with a melting point of 328C or zinc with a meltingpoint of4l9C. However, most metals, especially those which are mostinert and generally the most desirable, have very high melting pointsand usually require temperatures significantly higher than their meltingpoints to produce vapor pressures which are effective for coating. Forexample, aluminum, a highly inert and desirable metal, melts at 660C andrequires significantly higher temperatures to produce vapor pressuresuseful for coating, and tungsten, another desirable metal, melts at3370C. Not only do such high temperatures make deposition of the metalfrom the vapor of the molten metal impractical, but also these vaporswould be so hot as to significantly deteriorate the properties of thepresent magnetic transition metal-rare earth alloy powders.

Similarly, the coating of cobalt-rare earth particles by electrolessplating techniques is not highly attractive since these methods requireplacing the very fine, generally 10 micron average particle size, andconsequently very reactive cobalt-rare earth powders into contact withan aqueous solution which is highly acidic and results in thedissolution of significant amounts of material. These plating techniquesalso do not appear to produce a continuous uniform coating on these fineparticles. In addition, long term deleterious effects on the magneticproperties of the cobalt-rare earth powders can be expected from thedirect effects of the acidic aqueous solutions or from amounts of waterentrapped within the metal coating in the thin layer of Co and Sm Osurrounding each particle which reacts slowly with the base cobalt-rareearth alloy.

The process of the present invention overcomes the disadvantages of theprior art and provides a solution to the oxidation problem of thesereactive materials which obviates the sintering procedure by coating thepowders with a coherent and non-reactive material without significantlyaffecting the magnetic properties of the powder. In the present processa metal coating is deposited on the powder by a metal vapor produced bythe decomposition of an organometallie compound.

Briefly stated, the process of the present invention comprises providingparticles of a magnetic transition metal-rare earth alloy, heating anorganometallic compound to decompose said compound and produce a metalvapor, and contacting said metal vapor with said particles to deposit acoating of metal thereon.

The air-stable product of the present invention is comprised ofparticles of a transition metal-rare earth alloy enveloped by a coatingof a material impervious to air, said material being selected from thegroup consisting of a vapor-deposited metal having a melting pointhigher than 500C or an oxide of said vapordeposited metal.

In the present process a magnetic transition metalrare earth alloy,e.g., TRE, where T is a transition metal and RE is a rare earth metal,is used in particle form. The transition metal is selected from thegroup consisting of cobalt, iron, nickel, manganese and alloys thereof.

The rare earth metals useful in the present process are the 15 elementsof the lanthanide series having atomic numbers 57 to 71 inclusive. Theelement yttrium (atomic number 39) is commonly included in this group ofmetals and, in this specification, is considered a rare earth metal. Aplurality of rare earth metals can also be used to form the presentintermetallic compounds which, for example may be ternary, quartenary orwhich may contain an even greater number of rear earth metals asdesired. Mischmetal, an abundant common alloy of rare earth metals, isparticularly advantageous.

Representative of the cobalt-rare earth compounds useful in the presentinvention are cobaltcerium, cobaltpraseodymium, cobalt-neodymium,cobaltpromethium, cobaltsamarium, cobalt-europium, cobalt-gadolinium,cobalterbium, cobaltthulium, cobalt-ytterbium, cobalt-lutecium,cobalt-yttrium, cobaltlanthanum and cobalt-mischmetal. Examples ofspecific ternary compounds include cobalt-ceriumpraseodymium,cobalt-yttrium-praseodymium, and cobalt-praseodymium-mischmetal.

Transition metal-rare earth intermetallic alloys or compounds exist in avariety of phases and each phase may vary in composition. A materialsubstantially comprised of the T RE single phase is particularlypreferred in the present invention since this phase has shown the mostdesirable combination of magnetic properties.

The transition metal-rare earth compound or alloy of the present processcan be prepared by a number of methods. For example, it can be preparedby melting the transition metal and rare earth metal together in theproper amounts under a substantially inert atmosphere such as argon andallowing the melt to solidify.

The alloy can be converted to particulate form in a conventional manner.For example, it can be crushed to a coarse size and then pulverized to afiner form by, for example, fluid energy milling in a substantiallyinert atmosphere. Alternatively, the powder can be produced initially bya reduction-diffusion process as set forth in copending application Ser.No. 172,290, filed on Aug. 16, l97l in the name of Robert E. Cech, nowU.S. Pat. No. 3,748,193. Also, in some instances, it may be desirable togrind sintered compacts of these powders to a desired particle size.

The particle size of the transition metal-rare earth alloy used in thepresent process may vary. It can be in as finely divided a form asdesired. For best magnetic properties, average particle size will rangefrom about 1 micron or less to about microns. Larger sized particles canbe used, but as the particle size is increased, the maximum coerciveforce obtainable is lower because the coercive force decreases withincreasing particle size.

In the present process, an organometallic compound is used which can bea solid, liquid or gas at room temperature and which decomposes attemperatures lower than 500C to produce a metal vapor. The metal vaporis contacted with the present TRE alloy powder to deposit metal thereonto form a continuous coating of metal which protects the powder from theatmosphere. The present coating process yields a coherent substantiallyuniform layer of metal and is accomplished in an atmosphere in which thereactants are inert. Typical inert atmospheres which are suitable in thepresent invention include argon, nitrogen or a vacuum. No water vapor oroxygen gas is present to degrade the magnetic properties of the alloymaterials.

In carrying out the present process, the organometallic compound and theTRE alloy powder are preferably admixed to produce a substantiallyintimate mixture so that when the organometallic is decomposed, theresulting metal vapor, which deposits metal on contact with the surfaceof the alloy powder, will be distributed substantially uniformlythroughout the powder to effectively deposit a continuous coatingthereon and thereby provide a barrier to the atmosphere. If desired,mixing can be continued during decomposition of the organometallic tomaintain a substantially intimate mixture. When the organometalliccompound is a solid at room temperature, it is preferably used in a finepowder form in order that it can form an intimate mixture with the TREalloy powder. When the organometallic is a liquid at room temperature,it should be admixed with the alloy powder to thoroughly wet thesurfaces thereof. Alternatively, the organometallic compound may bevaporized and flowed through the alloy particles in such form. In yetanother technique, the organometallic can be decomposed and theresulting metal vapor carried by an inert gas, such as argon, intocontact with the alloy powder to deposit a coating thereon.

In some instances the uniform deposition of a metallic coating on TREalloy powder may be hindered because the organometallic is notferromagnetic in nature while the TRE alloy powder is. This problem mayparticularly occur when the organometallic compound is a solid at roomtemperature and when the organometallic and TRE alloy powder are simplymixed together prior to and/or during the coating process itself. Due totheir magnetic nature, the TRE alloy powders may tend to conglomerateand so separate from the organometallic compound resulting in thedeposition of an uneven metallic coating. The solution to this problemis to use a liquid organic carrier compound which will-not attack and somagnetically degrade the TRE alloy powders and which will dissolve atleast to a minor extent the organometallic desired for the coatingoperation. The organic carrier liquid can then be used to eitherdissolve or to form a slurry with the desired organometallic. Then thissolution or slurry can be admixed with the TRE alloy powders in aconventional manner, e.g., either mechanically or magnetically agitated,to produce a substantially intimate mixture. The organic carrier liquidcan then be removed by either gentle heating or the application of avacuum or a combination of both of these. The organometallic willprecipitate from the solution or slurry and coat the TRE alloy particlessubstantially uniformly. The TRE alloy particles now coated with anorganometallic powder can then be heated under the appropriateconditions to decompose the organometallic and leave the desiredmetallic coating. Representative of the organic liquid carriers usefulin the present invention are carbon tetrachloride; l,l1-trichlorotrichloroethylene; l ,l,1trichloroethane and dimethylsulfoxide.

A number of conventional techniques can be used to carry out the presentprocess. However, best results are obtained by the use of a fluid bedreactor supporting the transition metal-rare earth alloy powder acrosswhich flows an inert gas stream bearing a significant partial pressureof the organometallic compound to be decomposed. Around the fluid bed isa furnace supplying a sufficient amount of heat to decompose theorganometallic compound in the gas phase to produce a metal vapor whichdeposits a metal coating on the powder particles. The partial pressureof the organometallic compound should be sufficient to yield, whendecomposed, a partial pressure of metal vapor sufficient to effectivelydeposit a metal coating on the particles which envelops the particles ina reasonable period of time, i.e., less than 8 hours. The particularuseful partial pressure of organometallic compound is determinableempirically and generally is at least about atmosphere.

Alternatively, another coating technique makes use of the magneticproperties of the particles themselves to help produce continuouscoatings. For example, a mixture of the transition metal-rare earthalloy powder and the desired organometallic is stirred in a nonmagneticcontainer by an external magnet while the temperature is raised to thepoint at which the organometallic decomposes producing a metal vaporwhich deposits metal on contact with the particles. In this em bodimentan inert gas stream is passed through the container or a vacuum ispulled on the container during processing.

In the present process, the amount of organometallic compound used isdeterminable empirically. It should be used in an amount which, ondecomposition, is sufficient to produce an amount of metal vapor whichcondenses on the exposed surfaces of the alloy particles to form acontinuous coating of metal thereby preventing penetration by theatmosphere. Specifically, the amount of organometallic compound usedshould, upon decomposition, yield a significant partial pressure ofmetal vapor, generally at least about 10 atmosphere, sufficient toeffectively coat the exposed surfaces of the alloy particles with acontinuous coating of metal. The organometallic compound may decomposeinitially to yield the metal vapor or it may decompose to yield anotherorganometallic vapor which is then decomposed to give the metal vapor.Preferably, the organometallic compound should be used in an amountwhich on decomposition, produces the metal in an amount ranging from 1to 5% by weight of the alloy powder. From formulas and atomic weights,the weight relationships between the substances in the reaction can becalculated readily.'Amounts of deposited metal less than 1% by weight ofthe alloy powder are likely to result in a discontinuous coating whereasamounts of deposited metal significantly greater than 5% by weight ofthe alloy powder will dilute the magnetic properties of the powder. Bestresults are attained with the metal being deposited in an amount of 2%by weight of the alloy powder.

The minimum thickness of the metal coating need only be sufficient tomake it continuous, e.g., at least a film-forming thickness which isabout one microinch, to prevent air from penetrating to the surface ofthe alloy particles. In some instances where a metal may form a porousoxide, thicker continuous coatings of the metal should be deposited tomake the outer portion of such a metal coating available to be oxidizedby the air leaving an inner continuous metal coating to maintain thestability of the magnetic properties of the alloy particles. However, anumber of metals, for example, aluminum, form non-porous oxides whichare effective barriers to air. Metal coatings significantly thicker thanthat necessary to provide the alloy particles with an effective barrierto the atmosphere provide no particular advantage since they do notimprove magnetic stability and prevent a close packing of the alloyparticles in the non-magnetic matrix thereby diluting the magneticproperties. Metal coatings thicker than necessary can be useful if suchmetal is also to serve as a matrix or partial matrix for supporting theparticles.

One method of determining that the continuous coating of metal has beenformed on the exposed surfaces of the alloy particles is to magnetizethe coated particles, measure their intrinsic coercive force at roomtemperature, heat the particles in air at an elevated temperature, forexample C, for a significant period of time, i.e., at least about 30minutes, and remeasure their intrinsic coercive force at roomtemperature. If the intrinsic coercive force of the coated particlesafter heating is not significantly lower than before the heating, theparticles can be considered as being effectively coated in accordancewith the present invention.

In the present invention the solid metal coating should have a number ofproperties. Specifically, it should provide a barrier to the atmosphere,and also, if desired, the metal coating can be chosen for some otherdesired property, e.g., ductility. The metal, itself, should have nosignificant deteriorating effect on the magnetic properties of thepowder. It should be nonmagnetic or so weakly magnetic as not todiminish the magnetic properties of the powder significantly.

In the present invention, the particular deposited coating can becomposed of more than one metal to form an alloy depending on theparticular properties desired. A plurality of metals, for example Cu andZn, can either be deposited sequentially or concommitantly in anyproportion to form an alloy coating on the particles.

The non-metallic products of decomposition are gaseous, or usuallyevaporate from the alloy particles during the decomposition step, or canbe evaporated therefrom at temperatures below 500C, such removal beingpreferably promoted by a flowing atmosphere or a substantial vacuum.Since the non-metallic products of decomposition are much less dense andsignificantly more easily vaporizable than the deposited metal, they donot interfere with the formation of continuous metal coatings in thepresent invention.

In the present process, there are a number of useful organometalliccompounds which decompose at temperatures below 500C. Typical of theseis triisobutylaluminum as a source of aluminum. Specifically, the metalcoating of aluminum can be deposited according to the followingreactions:

1. Al(CH CH mans- 2CH C(CI-I )2. This operation must be accomplished atreduced pressures because the triisobutylaluminum cant be successfullydistilled above 10mm Hg.

There are a number of advantages to the use of triisobutylaluminum andother organometallics which decompose in a similar manner. One advantageis that the low temperature at which this organometallic decomposes willnot affect the magnetic properties of the transition metal-rare earthalloy powder. Another advantage is that the amount of chemicalinteraction between the aluminum and the alloy powder should be minimalat these temperatures. Yet another advantage is that the hydrogen gaspresent can be expected to reduce any surface oxides present on thealloy particles. Also, although it has been reported that cobalt-rareearth alloy powder absorbs hydrogen, pressures of H somewhat in excessof 76 cm I-Ig are needed. Therefore this proposed process working with alow residual pressure, -l0mm Hg, should minimize deleterious effects dueto hydrogen absorption. In addition, the organometallic decompositionreaction is relatively clean and yields products, except for elementalAl, which are AlH(CH CH gases and are accordingly easily removed fromthe coated powders.

A typical example of an organometallic useful in the present inventionfor the vapor deposition of copper is phenylcopper, C H Cu, whichthermally degrades according to the following reaction:

This reaction affords advantages similar to those listed for the Aldeposition, and in addition, it takes place at a very low temperature.

Table 1 lists a partial series of elemental metals useful as coatingsand their organometallic compounds suitable for the present process. Inthe present invention, metal carbonyls are assumed to be organometalliccompounds.

TABLE 1 Metal Organometallic Compound Cu Copper formate. Cu(CH Copperacetylacetonate (Cu(CH COCHCOCl-LQ Methylcopper, CuCH Ni Nickelcarbonyl. Ni(CO) Fe lron carbonyl Fe(CO) Cr Chrominum carbonyl. Cr(CO).,

Bisbenzene chromium, Cr(C.,l-l Mo Molybdenum carbonyl, Mo(CO).,

Bisbenzene molybdenum. Mo(C,,H.;) Benzene molybdenum carbonyl. C HMo(CO);, W Dibenzene tungsten. W (Cd- .9

Mesitylene tungsten carbonyl. (CH C l-i W(CO) Tungsten carbonyl. W(CO).,Ru Ruthenium carbonyl. Ru(CO) and/0r Ru (CO) lr Iridium carbonyl. Ir(CO) V Vanadium carbonyl. V(CO).,

Bisbenzene vanadium. (C H hV Hf Dicyclopentadienyl hafnium dichloride,

135): Hf Ta Tantalum methylcyclopentadienyl tetracarbonyl,

CH C H Ta(CO), Nb Niobium methylcyclopendadienyl tetracarbonyl,

CH C H NMCO), Zn Diethylzinc, Zn(C H Dimethyl zinc, Zn(Cl-l 5 Zincacetylacetonate (Zn(CH COCHCOCl-l Be Diethyl beryllium, (C H Be MgDiphenylmagnesium. Mg(C,;H

Diethylmagnesium. Mg(C H Sn Tetramethyl tin. Sn(CH Bi Trimethyl bismuth.Bi(CH Au Diethyl gold bromide. ((C H AuBi) Pb Tetraethyl lead. Pb(C H MnDicyclopentadienyl manganese. (C H ,Mn Re Rhenium carbonyl, Re (CO) RhRhodium carbonyl. Rh (CO),. Ti Dicyclopentadienyl titanium, (C H Ti Inaddition to the above organometallics listed in Table I, there are anumber of trifluoroacetylacetonates and hexafluoroacetylacetonates ofvarious metals, e.g., Zn and Zr which could yield the desired metalcoating. Hydrates of some of the above organometallics can be used inplace of the anhydrous compounds in which case the water of hydrationwill be expected to be removed rapidly on heating, e.g., by a flowinginert gas stream or a vacuum.

The present metal-coated particles are useful in the manufacture ofmagnets which are air-stable, e.g., their magnetic properties do notdeteriorate significantly in air, at room temperature as well as atelevated temperatures which do not affect the barrier coatingsignificantly. Specifically, the coated alloy particles of the presentinvention can be incorporated in a nonmagnetic matrix to form magnets.The coated particles can be magnetized before or after incorporation inthe non-magnetic matrix, as desired, to produce the magnet.

The non-magnetic matrix used in forming the magnets of the presentinvention can vary widely. It can be, for example, a plastic or resin,an elastomer, or rubber, or a non-magnetic metal such as, for example,lead, tin, zinc, copper or aluminum. The extent to which the coatedalloy particles are packed in the matrix depends largely upon theparticular magnet properties desired.

Magnets having useful magnetic properties for a wide range ofapplications are produced when the coated alloy particles of the presentinvention are incorporated in a nonmagnetic matrix and magnetized. Themagnets of the present invention are useful in tele phones, electricclocks, radios,television, and phonographs. They are also useful inportable appliances, such as electric toothbrushes and electric knives,and to operate automobile accessories. In industrial equipment, thepresent magnets can be used in such diverse applications as meters andinstruments, magnetic separators, computers and microwave devices.

All parts and percentages used herein are by weight unless otherwisenoted.

In the following examples the intrinsic coercive force of each samplewas measured at room temperature. Specifically, a specimen of the powderwas prepared for magnetic measurement by introducing it into a body ofmolten paraffin wax in a small glass tube and cooling the wax in analigning magnetic field of 20,000 oersteds to align the particles alongtheir easy axis until the paraffin solidified. The intrinsic coerciveforce of the sample was then measured after applying a magnetizing fieldof 30,000 oersteds.

EXAMPLE 1 A sintered body of compacted CoSm alloy powder, preparedsubstantially as set forth in U.S. Pat. No. 3,655,464, was ground to apowder using a jaw crusher and a jet mill. The alloy powder wascomprised substantially of Co Sm phase and a minor amount of Co Smphase. Approximately 5 grams of the alloy powder having a size of 325mesh (U.S. Standard Screen Size), e.g., and average particle size ofabout 6 microns, were placed in a U-tube along with about 1 gram ofcopper acetylacetonate, present as the hydrate Cu(CH COCHCOCI-I .2H O,and in powder form which was calculated to yield copper in an amount ofabout 2% by weight of the alloy powder. A slow stream of argon gas waspassedthrough the U-tube across the mixture which was mixed by moving amagnet beneath it. After a few minutes, a period of time consideredsuitable for the removal of the majority of oxygen present by the argonstream, the mixture was first gently heated with Meeker burner andoccasionally during the entire heating process a magnet was used to stirup the mixture to assist both in driving off the water of hydration andin obtaining a more uniform metal coating on the alloy particles. Thetemperature of the mixture was then raised slowly to about 400C toaccelerate the decomposition of the copper acetylacetonate.

After a minute or two a brown coating was observed being deposited onthe alloy particles and on the walls of the U-tube. Heating wascontinued for several more minutes then stopped. The coated alloyparticles were allowed to cool to room temperature with argon continuingto flow over them. After the powders were cool a magnet was used toseparate the coated alloy particles from a small amount of non-magneticmaterial present, presumably unreacted copper acetylacetonate andelemental Cu metal.

The alloy particles were examined under an optical microscope and byscanning electron microscopy. They appeared to be enveloped by acontinuous uniform brown coating of copper metal.

The intrinsic coercive force, H of a portion of these copper-coatedparticles, as well as of a portion of the uncoated particles of the samecomposition and size, was determined and the results are shown in RunNo.

l of Table II.

The copper-coated particles, as well as the uncoated particles of thesame composition and size, were placed in an oven having an airatmosphere and maintained at a temperature of 92C. At the end of eachperiod of time indicated in Table II, a portion of the copper coatedparticles as well as a portion of the uncoated particles were removedfrom the oven and cooled in air to room temperature. The intrinsiccoercive force, l-l of each such portion was then determined and theresults are shown in Table II.

As shown by Table II, the copper-coated particles had an intrinsiccoercive force initially higher than that of the uncoated particles, andafter being heated at 92C in air for periods of time as long as l 12hours, the intrinsic coercive force of the copper-coated particlesimproved significantly whereas that of the uncoated particlesdeteriorated significantly. This indicates that the copper coatingprovided an effective barrier to the atmosphere to prevent atmosphericoxygen from reacting with and degrading the magnetic properties of the CSm powder. In addition, the significant increase in intrinsic coerciveforce of the copper-coated particles after heating may be due to removalof sites for reverse domain initiation on the now protected surface ofthe alloy particles.

EXAMPLE 2 Copper-coated Co Sm alloy particles were prepared as set forthin Example 1. Solid polyproplene powder in an amount of about 3.5% byweight of the coppercoated alloy particles was admixed therewith and themixture was placed in a die press. An aligning magnetizing field of20,000 oersteds was applied to the mixture to align the alloy particlesalong their easy axis during pressing. The temperature of the press wasraised to 200C to liquify the polyproplene and a pressure of 100,000 psiwas applied at this temperature for about 5 minutes. The press was thenallowed to cool to room temperature in the magnetizing aligning field.

The resulting solid compact was removed from the press and wasmagnetized in a field of 60,000 kilooersteds. Its energy product wasdetermined to be 4 MGOe.

EXAMPLE 3 In this example the cobalt-samarium alloy powder used wassubstantially the same as that set forth in Example l. The intrinsiccoercive force of portions of the powder was determined before and afterannealing in an air oven at C for 30 minutes and the results are shownin Run No. 6 of Table III.

Portions of the alloy powder were coated with metal substantially as setforth in Example 1. Specifically, chromium hexacarbonyl, Cr(CO) in anamount of about 4% by weight of the alloy powder, was admixed therewithand the mixture placed in a U-tube where it was maintained under astream of argon and substantially continuously mixed with a magnet. Itwas calculated that in this mixture chromium hexacarbonyl would yieldchromium in an amount of 1% by weight of the alloy powder. At atemperature of 400C the chromium hexacarbonyl decomposed, and afterabout a minute, a silvery coating was observed on the alloy powder andon the walls of the tube. Heating and mix ing was continued for about 10additional minutes to insure complete coating of the particles withchromium and then stopped. The chromiumcoated alloy particles werecooled to room temperature under argon. A magnet was then used toseparate the coated particles from a small amount of non-magneticmaterial present. The intrinsic coercive force of a portion of thecoated powder was determined and is shown in Run No. 7. The remainingcoated powder was heated in an air oven at 150C for 30 minutes, thencooled to room temperature and its intrinsic coercive force determinedas shown in Run No. 7 of Table III.

The procedure used in Run No. 8 was the same as that of Run No. 7 exceptthat chromium hexacarbonyl was used in an amount of 20% by weight of thealloy powder which was calculated to yield chromium in an amount of 5%by weight of the alloy powder.

In Run No. 9 triisobutylaluminum was used in an amount which coated thealloy particles with aluminum in an amount of about 2% by weight of thealloy powder. The coating procedure differed from Run No. 7 in that adouble U-tube was used wherein the triisobutylaluminum was placed in oneUcurve and the alloy powder was placed in the second U-curve. Thetriisobutylaluminum was decomposed at a temperature of 250C and theresulting aluminum vapor was carried by the argon stream into contactwith the alloy powder where it condensed on the powder which was beingcontinuously mixed with a magnet to insure the deposition of acontinuous uniform coating of aluminum.

The metal-coated particles of Runs 7-9 were examined under an opticalmicroscope and a scanning electron microscope. They appeared to becompletely and substantially uniformly coated by metal.

The intrinsic coercive forces of the uncoated and coated powders, beforeand after annealing in air, are given in Table III.

As illustrated by Table III, the intrinsic coercive force of Run No. 6,the uncoated control powder, deteriorated significantly after 30 minutesat 150C whereas the intrinsic coercive force of the metal coatedparticles of Run No. 7 through 9, which illustrate the presentinvention, increased significantly after the annealing treatment in air.This indicates that the present metal-coated particles provided aneffective barrier to the atmosphere to prevent atmospheric oxygen andmoisture from reacting with and deteriorating the magnetic properties ofthe present alloy powder.

In copending U.S. Pat. application Ser. No. 372,688 now U.S. Pat. No.3,856,581, entitled Annealed Air- Stable Magnetic Materials HavingSuperior Magnetic Characteristics And Method" filed of even dateherewith in the names of Richard J. Charles and John G. Smeggil there isdisclosed a process for producing air stable coated particles of amagnetic material which comprises providing particles of a transitionmetal-rare earth alloy andan organometallic compound which decomposes ata temperature lower than 500C, heating the organometallic compound todecompose it to produce a metal vapor, contacting the metal vapor withthe particles of transition metal-rare earth alloy to deposit a metalcoating thereon which substantially envelops theparticles, and heatingthe metal coated particles at a temperature ranging from about 50C to200C to increase their intrinsic coercive force by at least 10%.

In copending U.S. Pat. application Ser. No. 372,690 now US. Pat. No.3,856,582, entitled Fabrication of Matrix Bonded Transition Metal-RareEarth Alloy Magnets filed of even date herewith in the names of RichardJ. Charles and John G. Smeggil there is disclosed a process forproducing an air-stable porous magnetic compact which comprises admixingparticles of a transition metal-rare earth alloy with an organometalliccompound which decomposes at a temperature below 500C and pressing themixture to form a green body. The green body is heated to decompose theorganometallic compound to produce a non-metallic product and a metalvapor. The metal vapor deposits an interconnecting continuous coating ofmetal on the exposed surfaces of the pressed alloy particles therebypreventing penetration by the atmosphere, and the non-metallic productis outgassed from the body leaving the resulting coated compact porous.

All of the above cited applications are, by reference, made part of thedisclosure of the present application.

What is claimed is:

1. An air-stable product for the manufacture of a permanent magnet, saidproduct being comprised of particles of cobalt-rare earth alloyenveloped by a coating impervious to air, said coating being selectedfrom the group consisting of a vapor-deposited metal having a meltingpoint higher than 500C or an oxide of said vapor-deposited metal, saidproduct being produced by a process for coating a metal having a meltingpoint above 500C on cobalt-rare earth alloy particles withoutsignificantly deteriorating their permanent magnet properties producingmagnetically air-stable particles which comprises providing particles ofcobalt-rare earth alloy having an average size up to about 10 microns,providing an organometallic compound which at a temperature below 500Cdecomposes and yields products of decomposition consisting of gaseousnonmetallic product and a metal vapor, placing said compound and saidparticles in a substantially inert atmosphere which is a flowingatmosphere or a substantial vacuum, heating said organometallic compoundat a temperature below 500C and substantially completely decomposing itand producing said gaseous product of decomposition and a metal vapor,contacting the resulting metal vapor with the cobaltrare earth alloyparticles depositing a coherent substantially uniform metal coatingwhich at least envelops the particles providing an effective barrier tothe atmosphere and which has no significant deteriorating effect ontheir magnetic properties and diffusing away the non-metallic gaseousproducts of decomposition, said organometallic compound being used in anamount which on decomposition yields a partial pressure of metal vaporof at least about 10" atmosphere and produces the metal in an amountranging from 1 to 5% by weight of said cobaltrare earth alloy particlesand said deposited metal having a melting point above 500C.

2. An air-stable product according to claim 1 wherein saidvapor-deposited metal is aluminum.

3. An air-stable product according to claim 1 wherein saidvapor-deposited metal is copper.

4. An air-stable magnet having as the active magnetic component theparticles of claim 1.

1. AN AIR-STABLE PRODUCT FOR THE MANUFACTURE OF A PERMANENT MAGNET, SAIDPRODUCT BEING COMPRISED OF PARTICLES OF COBALTRARE EARTH ALLOY ENVELOPEDBY A COATING IMPERVIOUS TO AIR, SAID COATING BEING SELECTED FROM THEGROUP CONSISTING OF A VAPORDEPOSITED METAL HAVING A MELTING POINT HIGHERTHAN 500*C OR AN OXIDE OF SAID VAPOR-DEPOSITED METAL, SAID PRODUCT BEINGPRODUCED BY A PROCESS FOR COATING A METAL HAVING A MELTING POINT ABOVE500*C ON COBALT-RARE EARTH ALLOY PARTICLES WITHOUT SIGNIFICANTLYDETERIORATING THEIR PERMANENT MAGNET PROPERTIES PRODUCING MAGNETICALLYAIR-STABLE PARTICLES WHICH COMPRISES PROVIDING PARTICLES OF COBALT-RAREEARTH ALLOY HAVING AN AVERAGE SIZE UP TO ABOUT 10 MICRONS, PROVIDING ANORGANOMETALLIC COMPOUND WHICH AT A TEMPERATURE BELOW 500*C DECOMPOSESAND YIELDS PRODUCTS OF DECOMPOSITION CONSISTING OF GASEOUS NON-METALLICPRODUCT AND A METAL VAPOR, PLACING SAID COMPOUND AND SAID PARTICLES IN ASUBSTANTIALLY INERT ATMOSPHERE WHICH IS A FLOWING ATMOSPHERE OR ASUBSTANTIAL VACUUM, HEATING SAID ORGANOMETALLIC COMPOUND AT ATEMPERATURE BELOW 500*C AND SUBSTANTIALLY COMPLETELY DECOMPOSING IT ANDPRODUCING SAID GASEOUS PRODUCT OF DECOMPOSITION AND A METAL VAPOR,CONTACTING THE RESULTING METAL VAPOR WITH THE COBALTRARE EARTH ALLOYPARTICLES DEPOSITING A COHERENT SUBSTANTIALLY UNIFORM METAL COATINGWHICH AT LEAST ENVELOPS THE PARTICLES PROVIDING AN EFFECTIVE BARRIER TOTHE ATMOSPHRE AND WHICH HAS NO SIGNIFICANT DETERIORATING EFFECT ON THEIRMAGNETIC PROPERTIES AN DIFFUSING AWAY THE NON-METALLIC GASEOUS PRODUCTSOF DECOMPOSITION, SAID ORGANOMETALLIC COMPOUND BEING USED IN AN AMOUNTWHICH ON DECOMPOSITION YIELDS A PARTIAL PRESSURE OF METAL VAPOR OFATLAST ABOUT 10**7 ATMOSPHERE AND PRODUCES THE METAL IN AN AMOUNT RANGINGFROM 1 TO 5% BY WEIGHT OF SAID COBALT-RARE EARTH ALLOY PARTICLES ANDSAID DEPOSITED METAL HAVING A MELTING POINT ABOVE 500*C.
 2. Anair-stable product according to claim 1 wherein said vapor-depositedmetal is aluminum.
 3. An air-stable product according to claim 1 whereinsaid vapor-deposited metal is copper.
 4. An air-stable magnet having asthe active magnetic component the particles of claim 1.